This invention relates to the production of iron from iron oxide in the form of fines or concentrates by agglomerating the iron oxide with a solid carbonaceous reductant, heating and reducing the agglomerates in a rotary hearth furnace, and smelting the reduced agglomerates.
The invention is more particularly described as a method of making iron from finely divided iron ores, by the steps of agglomerating finely divided iron ore to form compacts, prereducing the compacts in a rotary hearth furnace, discharging the prereduced compacts at a temperature of at least 1000.degree. C., and smelting and further reducing the prereduced compacts in a smelting reduction vessel to form a molten iron product.
Currently, commercial processes for direction reduction use a shaft furnace, fluidized bed furnace or a rotary kiln. Both the shaft furnace and rotary kiln require an indurated agglomerate or lump ore as feed material. Both the shaft furnace and the fluid bed furnace tend to have sticking or clustering problems, and rotary kilns having ringing problems in which the burden tends to adhere to the side of the kiln. These sticking problems are increased when heating to higher reduction temperatures. However, higher temperatures are preferable in order to increase the rate of reduction reactions. We have found that higher operating process temperatures (above the temperature at which particles stick together) are possible when utilizing a rotary hearth furnace for direct reduction or prereduction, since the particles on the rotary hearth are not in intimate contact, particularly when the bed is only one or two particles deep. The upper layer of particles is not sufficiently heavy to cause particles on the upper layer to stick to particles in the lower layer.
Rotary hearth furnace operation require a gas containing a high percentage of reductants (CO+H.sub.2). Such a gas is normally referred to as a high quality gas. "Quality" is defined as the ratio of reductants (CO+H.sub.2) to oxidants (CO.sub.2 +H.sub.2 O) in the gas. Rotary hearth furnace operation normally requires a high quality gas. However, in the present invention such a high quality gas is not required because the reductant, solid carbonaceous material, is contained in the agglomerate being reduced in the rotary hearth furnace. A gas quality of 2.4 is sufficient to prevent reoxidation of the reduced iron in the rotary hearth furnace, because such gas will be in equilibrium with iron at 1200.degree. C. Reduction of iron oxide in a rotary hearth furnace is fairly rapid because of the extremely high temperatures involved and the intimate contact of iron oxide with solid reductant in the agglomerated feed material on the hearth. When more than a single layer of agglomerated particles are reduced in a rotary hearth furnace, there is a marked degree of difference in reduction between the layers. The uppermost layer reaches reduction temperature first, and consequently achieves high metallization in a short period of time. Lower layers, being shielded from direct heat radiation, take a longer time to reach processing temperature, and therefore take a longer time to achieve the desired metallization. During the period of time after the top layer has achieved its desired metallization and while the lower layers are increasing in metallization is a critical time to the uppermost layer if the furnace gases or oxidizing to iron. This will cause a loss of metallization of the topmost layer while the lower layers are undergoing reduction. To prevent this loss in metallization in the uppermost layers, a rotary hearth furnace can be operated with burners adjusted for rich operation to produce a gas which is in equilibrium with metallic iron. Producing such gas is extremely difficult to achieve with coal in the burners. However, this atmosphere can be readily produced with a gaseous or liquid fuel and preheated air.
We have found that utilizing off-gas from a smelting reduction vessel in the rotary hearth will overcome the foregoing difficulties and allow the operation of the rotary hearth furnace with 100% off-gas rather than requiring additional gaseous or liquid fuels.